The present invention relates to a method and/or a device for modeling a hydraulic system having two peripheral volume storage vessels, each of which is connected to a central volume storage vessel by a valve, based on a hydraulic model, i.e., a method for estimating pressure and/or volume changes in this system in response to external changes.
Modeling methods of this type may be used in braking force control or anti-skid systems for motor vehicles. German Published Patent Application No. 43 40 921, German Published Patent Application No. 40 12 167, and German Published Patent Application No. 40 30 724 concern braking force control system of this type.
The brake system of a motor vehicle forms a hydraulic system with a brake master cylinder, to which a delivery pump may be connected, brake chambers in the vehicle wheels, and a cable system, referred to as the brake circuit, which includes a pressure line connecting a changeover valve at the output of the brake master cylinder to intake valves of the brake chambers. The wheel brake chambers can be viewed as peripheral volume storage vessels and the pressure line as a central volume storage vessel of the hydraulic system.
The braking force control system is an electronic arrangement that operates in cycles and uses sensors to detect the pressure in the brake master cylinder as well as the motive characteristics of the wheels, and, by driving the valves located in the brake circuit between the brake master cylinder and the wheel brake chambers, tries to set a braking pressure at which desired motive characteristics of the wheels, such as rotational speed or slip, can be achieved as accurately as possible. The valves connecting the brake master cylinder to the wheel brake chambers can be controlled independently of one another, and situations can arise in which the pressures present in the pressure line of the brake circuit and in two wheel brake chambers differ from one another.
To control the motive characteristics of the wheels in the desired manner, it must be possible to estimate the pressure building up in the wheel brake chambers as a result of a pressure change in the pressure line of the brake circuit. Because the control arrangement operates in cycles, it can detect new values for the parameters to be taken into account only at certain intervals.
The control action performed by the arrangement must remain oriented toward these parameter values until new values are available; in fact, the values of these parameters change continuously between two detection times. Systematic control errors are the result.
In a hydraulic system like the one illustrated schematically in FIG. 2, this has an effect in particular on the modeling of situations in which different pressures p1, p1 prevail in the wheel brake chambers, i.e., peripheral volume storage vessels 1, 2, where p1 less than p2 is assumed. When pressure p3 is present in the pressure line, i.e., in central volume storage vessel 3, at interval [p1, p2], the result in a real system is a volumetric flow through open valves 5, 6 from higher-pressure wheel brake chamber 2 to lower-pressure wheel brake chamber 1 via pressure line 3 located between them. Conventional hydraulic models are unable to account for this pressure. They calculate a volumetric flow q13 between storage vessels 1 and 3, based on pressure values p1 and p3 present at the beginning of a model duty cycle, as well as a volumetric flow q13 between storage vessels 2 and 3, based on pressure values p2 and p2. It is not possible to account for the influence of volumetric flow q13 on volumetric flow q23 and vice versa during the duty cycle.
The problem becomes particularly apparent, for example in the case of p1=p3 less than p2. In this situation, the conventional model does not show any volumetric exchange between storage vessels 1 and 3 during the first cycle. The model treats volumetric flow q23 from wheel brake chamber 2 to pressure line 3 as though it had remained stored in the pressure line during this cycle. The pressure increase that this produces in pressure line 3 does not yield a volumetric flow into wheel brake chamber 1 until the next cycle. At the same time, ignoring the volumetric outflow from pressure line 3 to wheel pressure chamber 1 means that an unrealistically high pressure is assumed in the central storage vessel during the next cycle, and the calculated volumetric flow from storage vessel 2 to storage vessel 3 is consequently too low. The model lags behind the actual situation, which impairs the quality of the model and consequently also the quality of a pressure control system in the brake chambers based on the model. This problem is particularly disturbing when the volume of the central storage vessel is smaller than the volume of the peripheral storage vessels. This occurs regularly when the central storage vessel is a line, as in the situation discussed here. In this case, namely, a very small exchanged volume is apparently sufficient to equalize the pressure difference between the peripheral volume storage vessel with the higher pressure and the central volume storage vessel in the model, and the calculation according to the model correspondingly supplies only a small value for the exchanged volume. The actual volume to be exchanged between the peripheral storage vessels to equalize the pressure can, however, be much higher, which means that the model moves toward correct pressure values only very slowly.
An object of the present invention is to provide a method and/or a device for estimating pressure and/or volume changes in a hydraulic system having two peripheral volume storage vessels connected to a central volume storage vessel, with the system being able to respond faster to pressure differences between the peripheral volume storage vessels than known methods. In particular, a method and/or a device is provided that makes it possible to account for such differences as early as the duty cycle in which they are first detected.
This object may be achieved by, using a method and/or a device for estimating pressure and/or volume changes based on a hydraulic model in a hydraulic system having two peripheral volume storage vessels, each of which is connected to a central volume storage vessel by a valve, the estimate is based at least at times on a model in which a direct connection to the peripheral volume storage vessels exists, regardless of whether the peripheral volume storage vessels are simultaneously connected directly to each other. This method can also be applied to hydraulic systems in which a direct connection between the peripheral storage vessels, e.g., using a valve, can be established as well as systems which do not have a connecting capability of this type.
The method is carried out in cycles, whereby preferably in each cycle:
a) the pressure is detected in each volume storage vessel;
b) for each peripheral volume storage vessel, a volumetric flow between this peripheral volume storage vessel and the central volume storage vessel is calculated on the basis of a pressure difference between this peripheral volume storage vessel and the central volume storage vessel;
c) if the direct connection between the peripheral volume storage vessels is used, a volumetric flow between the peripheral volume storage vessels is calculated on the basis of a pressure difference between the peripheral volume storage vessels;
d) the pressures at the end of the cycle are estimated on the basis of the calculated volumetric flows.
Each volumetric flow is suitably calculated in step b) on the basis of the volume coefficient or volume resistance of the valve through which the corresponding flow passes. The volumetric flow over the xe2x80x9cfictitiousxe2x80x9d connection between the peripheral volume storage vessels is calculated in step c), preferably on the basis of the total volume resistances of the valves connecting the peripheral volume storage vessels and the central volume storage vessel.